1. Field of the Invention
The present invention relates to a vibration reduction apparatus for reducing an image vibration and a camera system.
2. Description of the Related Art
Techniques for optically reducing an image vibration by a position control on an optical vibration reduction system or an image sensor are known.
For example, according to one conventional technique, a vibration of a camera (including an objective lens) is detected by an angular velocity sensor. Based on a detected angular velocity, the camera moves an optical vibration reduction system so as to cancel out a movement of an object image.
Japanese Unexamined Patent Application Publication Nos. Hei 10-322585 (hereinafter referred to as “Patent document 1”) and Hei 10-145662 (hereinafter referred to as “Patent document 2”) disclose a technique for suppressing an image vibration in a video camera. The video camera detects motion signals from shot images. Then, the video camera increases the sampling rate by interpolating the motion signals. The video camera increases the anti-vibration performance of the vibration reduction by feeding back an interpolated motion signal to a target drive position of an optical vibration reduction system.
[Problems of the Prior Art]
Incidentally, a sensor output of an angular velocity sensor contains such a component (hereinafter referred to as “reference value”) as a DC offset or a drift in addition to an angular velocity. To detect an image vibration correctly, it is necessary to eliminate the reference value from the sensor output carefully.
Usually, such a reference value varies with the temperature of the angular velocity sensor, the elapsed time, etc. in a complicated manner. Therefore, it is impossible to determine a reference value at the time of shipment from a factory.
In view of the above, conventionally, a method of separating and extracting a reference value from an output of the angular velocity sensor is employed. A camera shake caused by a human has dominant frequency components of 2-7 Hz. On the other hand, dominant frequency components of a reference value such as a DC offset or a drift are lower than about 1 Hz. Therefore, a reference value of a sensor output can be estimated by extracting low-frequency components that are lower than 1 Hz from the sensor output of the angular velocity sensor.
A true vibration component can be determined by eliminating (subtracting) the thus-estimated reference value from the sensor output.
However, because of the extraction of low-frequency components, this conventional method has various problems. For example, to extract low-frequency components of lower than 1 Hz from a sensor output, it is necessary to average a past sensor output for a very long period. Extracted low-frequency components have a long delay. Therefore, it is difficult to determine a reference value such as a DC offset or a drift in real time.
Further, part of the vibration component of a camera shake is not eliminated and remains as an error in extracted low-frequency components. If the low-frequency components including such an error are extracted from a sensor output as a reference value of the sensor output, an error is added to the true vibration component.
If vibration reduction is performed so as to cancel out such an error-added vibration component, the image drifts due to, for example, accumulation of errors. Or errors may cause a vibration.
As is understood from the above description, the anti-vibration performance of the vibration reduction depends on how to determine a reference value of a sensor output correctly.
[Problems of Patent Documents 1 and 2]
Incidentally, in Patent documents 1 and 2, a motion signal is fed back to the target drive position of the optical system (this control method is different in configuration from the control method of the present invention in which a motion signal is fed back to the reference value).
Where the control method of Patent documents 1 and 2 is applied to an electronic still camera, the following problems [1] and [2] arise.
[1] First, in electronic still cameras, motion signals are obtained from, for example, shot images for monitor display in a period before a manipulation on the release button. An average shooting interval (e.g., 30 frames/sec) employed in this case is several times to tens of times longer than the shooting interval (e.g., 60 fields/sec) of general video cameras. That is, in many cases, the sampling interval of motion signals in electronic still cameras is longer than in video cameras. In the conventional methods in which a motion signal of such a low frequency is fed back to the target drive position, non-negligible dead times occur and the target drive position follow-up performance and control stability are much lowered. Even an oscillation occurs in the worst case.
[2] In Patent documents 1 and 2, for matching with the target drive position update interval, a prediction value is generated by extrapolating motion signals.
In electronic still cameras, since motion signals have a long sampling interval, discontinuous interpolation errors produced by such extrapolative prediction are larger than in video cameras. Those interpolation errors become target drive position control errors as they are and hence much lower the anti-vibration performance.
Incidentally, in Patent documents 1 and 2, a high-pass filter is provided on a motion signal feedback path. Low-frequency components corresponding to a drift or an offset are cut by the high-pass filter. Therefore, in Patent documents 1 and 2, as a matter of fact, it is impossible to correct for a drift or an offset in a low-frequency range.
[Problems Relating to Motion Signal]
Further, the present inventors realized that the anti-vibration performance is lowered if a motion signal is fed back unconditionally.
For example, in electronic still cameras, the exposure time of shot images (what is called through images or the like) varies to a large extent depending on the field brightness. In this case, the shooting interval also varies, as a result of which motion signals vary.
Further, for example, in a state that vibration reduction is not effective, shot images blur greatly being influenced by a camera shake. In this state, the amplitudes of motion signals are large. On the other hand, the blur of shot images reduces quickly after a start of vibration reduction. Therefore, a large variation occurs between motion signals before and after the start of driving for the vibration reduction.
If motion signals having such a variation (disorder) are fed back as they are, the vibration reduction control is disturbed and the anti-vibration performance is lowered.